1. Field of the Invention
The present invention relates to an electric cable including a multilayer insulation made from silicone rubber, as well as a manufacturing process of said electric cable.
More particularly, it applies typically, but not exclusively, to the fields of power cables, such as medium voltage (especially from 5 kV to 45-60 kV) or high voltage (especially greater than 60 kV, which may be up to 800 kV) power cables, whether they are direct voltage (DC) or alternative voltage (AC) cables.
2. Description of Related Art
Medium voltage or high voltage power cables typically comprise a central electric conductor and, successively and coaxially around this electric conductor, a semiconducting inner layer, an electrically insulating (intermediate) layer and a semiconducting outer layer. These three layers can be crosslinked via techniques that are well known to those skilled in the art.
GB 870 583 describes a 3-layer crosslinked insulation for an electric cable, comprising a semiconducting inner layer, an electrically insulating (intermediate) layer and a semiconducting outer layer. Said three layers are made from vinyl-containing silicone gum. The semiconducting layers of said electric cable are made from a composition comprising vinyl-containing silicone gum, an organic peroxide as crosslinker, and acetylene black as conductive filler.
However, this process is not optimized to reduce significantly partial discharges between the electrical insulating layer and the semiconducting layers, when a voltage level of at least 5 kV is applied to the electric cable. Indeed, the use of conductive filler of carbon black type (e.g. acetylene black) can involve the formation of gas bubbles at the interface between the semiconducting layer and the electrically insulating layer. Said bubbles can be formed during the fabrication of the semiconducting layer, wherein carbon black can react with the silicone gum and/or the crosslinker, in forming said bubbles.